linux-kernel - Re: [PATCH 1/1] x86/vdso: fix a reference to a non-existing instruction

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Date:	Thu, 5 Mar 2015 15:22:15 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jiri Slaby <jslaby@...e.cz>, Jan Beulich <jbeulich@...e.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	rguenther@...e.de, stable <stable@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 1/1] x86/vdso: fix a reference to a non-existing instruction

On Thu, Mar 5, 2015 at 12:13 AM, Jiri Slaby <jslaby@...e.cz> wrote:
> .LSTARTFDEDLSI1 says:
>         /* HACK: The dwarf2 unwind routines will subtract 1 from the
>            return address to get an address in the middle of the
>            presumed call instruction.  Since we didn't get here via
>            a call, we need to include the nop before the real start
>            to make up for it.  */
>         .long .LSTART_sigreturn-1-.     /* PC-relative start address */
>
> But commit 69d0627a7f6e891189124d784d2fa90cae7c449a (x86 vDSO: reorder
> vdso32 code) from 2.6.25 replaced
>   .org __kernel_vsyscall+32,0x90
> by ALIGN right before __kernel_sigreturn. Of course, ALIGN need not
> generate any nop in there. Esp. gcc5 collapses vclock_gettime.o and
> int80.o together with no generated nops as "ALIGN". In that case
> kernel does not even link:
> ld: .eh_frame_hdr table[4] FDE at 0000000000000648 overlaps table[5] FDE at 0000000000000670.
>
> So fix this by adding there at least a single nop and make the
> function ALIGN possibly with more nops then.
>
> Kudos for reporting and diagnosing should go to Richard.
>
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Reported-by: Richard Biener <rguenther@...e.de>
> Cc: stable@...r.kernel.org
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> ---
>  arch/x86/vdso/vdso32/sigreturn.S | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/vdso/vdso32/sigreturn.S b/arch/x86/vdso/vdso32/sigreturn.S
> index 31776d0efc8c..d7ec4e251c0a 100644
> --- a/arch/x86/vdso/vdso32/sigreturn.S
> +++ b/arch/x86/vdso/vdso32/sigreturn.S
> @@ -17,6 +17,7 @@
>         .text
>         .globl __kernel_sigreturn
>         .type __kernel_sigreturn,@function
> +       nop /* this guy is needed for .LSTARTFDEDLSI1 below (watch for HACK) */
>         ALIGN
>  __kernel_sigreturn:
>  .LSTART_sigreturn:

Acked-by: Andy Lutomirski <luto@...capital.net>

Ingo or Thomas, can you apply this to tip:x86/urgent for this weekend?

Are we willing to depend on cfi-capable assemblers for 32-bit?  If so,
could we rewrite this garbage with .cfi directives?  The current code
is incomprehensible.

--Andy
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